1. Field of the Invention
The invention relates to a backlight unit, and more particularly to a backlight unit for improving assembly efficiency, eliminating light leakage, and reducing the thickness of backlight unit.
2. Description of the Related Art
FIG. 1A is a perspective view showing a conventional backlight unit for a liquid crystal display. As shown in FIG. 1A, a backlight unit includes a frame 10, a light guide plate 40, and a reflector 30. The side plates 32 of the reflector 30 are assembled with the frame 10 by positioning the screws 35 into holes (not shown) of the frame 10.
FIG. 1B is an exploded drawing of FIG. 1A. As shown in FIG. 1B, a reflecting sheet 20 is installed on the bottom surface of the light guide plate 40. The light guide plate 40 is positioned on the reflecting sheet 20. The light guide plate 40 has a pair of projection 42, the projections 42 will be inserted into the grooves 12 of the frame 10. A lamp 34 is located in the reflector 30, and the side plates 32 of the reflector 30 are assembled with the frame 10 by the screws 35 and the holes 15 of the frame 10. Further, a diffusing means 50 is installed above the light guide plate 40, and the installation of the backlight unit is finished. Generally, the diffusing means 50 includes a diffusing sheet 52, at least one prism sheet 54, and a protecting film 56. The protecting film 56 can also be used as a second diffusing sheet. Finally, a display panel, such as a liquid crystal panel 60, is installed above the backlight unit, and the installation of a display is finished.
In a liquid crystal display, the diffusing means and the light guide plate can be used for improving the luminance uniformity. In addition, the inserting dimension between the lamp and the light guide plate can also affect the luminance uniformity of the LCD. When the inserting dimension between the lamp and the light guide plate is too short, a light line will appear on the LCD (known as “Kido Mura” phenomenon), and the light guide plate will be overheated and damaged. When the inserting dimension between the lamp and the light guide plate is too large, a dark line will appear on the LCD, and the light guide plate will be loosed from the reflector easily. In the prior art, both the lamp 34 and the light guide plate 40 are fixed to the frame 10. During the usage of the LCD, the lamp 34 provides not only light but also heat. A part of the light guide plate 40 near the lamp 34 will expand at hot condition and shrink at cold condition. Therefore, the precision of the inserting dimension between the lamp and the light guide plate will be reduced, and also decreasing the tolerance between the frame and the light guide plate. Therefore, light lines or dark lines are prone to appear on the conventional backlight unit.
U.S. Pat. No. 5,815,227 discloses a backlight unit for a liquid crystal display. As shown in FIG. 2, at least two locking protrusions 44 are formed on the light guide plate 40, and at least two locking holes 36 are formed on the reflectors 30. After the locking protrusions 44 are inserted into the locking holes 36, the light guide plate 40 is directly coupled with the reflector 30. Therefore, the precision of the inserting dimension between the lamp and the light guide plate is improved.
However, the locking protrusions 44 of the light guide plate 40 increase not only the thickness of the backlight unit, but also the thickness of the liquid crystal display. Moreover, light will emit around the locking protrusions 44, so light leakage will occur at the display area of the display panel. In order to reduce the light leakage, the locking protrusion 44 is positioned away from the display area by increasing the size of the light guide plate 40, or the locking protrusion 44 is covered by a non-transparent component. Unfortunately, the methods will increase the area and/or thickness of the display. The prior arts are unable to manufacture a light and compact liquid crystal display.